Over the last 10,000 years Drosophila melanogaster and D. simulans have spread through the world in the wake of human migration. As a consequence, these now cosmopolitan species have been exposed to a variety of novel environments and habitats. For flies living in temperate environments, winter cold represents a novel environment and these species have evolved in response to cold temperatures in several distinct ways. D. melanogaster has evolved a diapause syndrome that confers extreme lifespan extension, reduction in metabolic rate, and elevated stress tolerance; although diapause increases winter survival, individuals able to diapause suffer a fitness disadvantage during the summer and thus this adaptation remains at intermediate frequencies even at high latitudes. D. simulans, on the other hand, appears to have a more modest adaptive response to winter conditions: this species performs well at cool temperatures but does not appear have the capacity to survive prolonged exposure to the harsh temperate winter. Little is known about other overwintering mechanisms in D. simulans, or indeed any evolutionary response to novel temperate environments. Herein, we propose to identify polymorphisms underlying adaptation to temperate climates in D. melanogaster and D. simulans, link these polymorphisms to function and test hypotheses about the evolutionary origin of these polymorphisms. To do this, we will collect large samples of individuals from both of these species (i) across latitude on the East and West coasts of North America, (ii) through the growing season at several sites near Philadelphia, PA and (iii) along an altitudinal transect in Northern California for four replicate years. First, we will identify polymorphisms that vary in consistent fashion through time and space through high- throughput sequencing technologies and we will verify changes in allele frequency through pyrosequencing. Second, we will identify the functional consequences of a subset of these polymorphisms using quantitative genetic and transcriptomic techniques. We hypothesize that many of the polymorphisms we identify will be associated with phenotypes known to vary in a clinal fashion amongst many drosophilid flies. Finally, we will test hypotheses about the evolutionary origin of these polymorphisms by assessing worldwide haplotype diversity at surrounding loci. We hypothesize that most of these adaptive alleles will be subject to soft- sweeps which are characteristic of species with extremely large population sizes or those with large amounts of genetic variation in ancestral populations such as both D. melanogaster and D. simulans.